The location of the portion of the cell cycle which requires anchorage dependence in normal cultured cells was determined using cells grown in soft agar, methylcellulose, and on Teflon. The block occurs during cytokinesis or early Gl phase. A mathematical method was devised for quantitating density dependent inhibition of proliferation in tumor cells which exhibit a density effect but do not reach a stationary saturation density. Specific suppression of the anti-tumor immune response as measured by a radioisotopic footpad assay was produced by the intraperitoneal inoculation of tumor homogenate. Selective growth of tumor cells on Teflon was demonstrated, providing a useful system for studying anchorage dependence of normal cells and for assaying tumorigenic potential in vitro.